This invention relates to liquid crystal displays, and more particularly to a method to stabilize a carbon alignment layer for the liquid crystals.
The alignment of liquid crystals (LC) in today""s flat panel liquid crystal displays (LCD) is accomplished by placing a thin film of LCs on a mechanically rubbed polyimide film coated on a suitable substrate, such as an indium tin oxide (ITO) film formed on glass. Limitations imposed by the mechanical rubbing method (e.g., creating multiple domains for improving the viewing angle) in conjunction with the difficulty of optimizing polymer materials (e.g., polymers that avoid image sticking) make it highly desirable to use alternative materials and a non-contact LC alignment method. There are a number of different methods/materials which have been shown to create LC alignment besides rubbing, such as (i) stretching of a polymer, (ii) a Langmuir Blodgeft film, (iii) a grating structure produced by microlithography, (iv) oblique angle deposition of SiOx, and (v) polarized UV irradiation of a polymer film.
More recently a viable non-contact method based on ion beam irradiation of a polyimide surface has been developed, as described in IBM""s patent U.S. Pat. No. 5,770,826. The method involves placing the LCs on a polyimide surface which has been bombarded with low energy (about 100eV) Ar+ ions. This process has many characteristics which make it suitable for the manufacture of LC displays. This method has been extended to include alignment layers of xe2x80x9cdiamond-likexe2x80x9d carbon (DLC), amorphous hydrogenated silicon, SiC, SiO2, glass, Si3N4, Al2O3, CeO2, SnO2, and ZnTiO2 films, as described in IBM""s patent U.S. Pat. No. 6,020,946.
IBM""s patent U.S. Pat. No. 6,061,114 describes a liquid crystal alignment layer made by directional deposition of a carbon film. In this case there is no need for a separate ion beam irradiation step.
One problem that has been discovered with respect to the ion beam irradiation method for LC alignment is that the anchoring energy of the LCs on ion beam treated DLC surfaces is not stable, but decreases substantially over a period of a few weeks.
What is needed is a method for LC alignment on DLC that allows the use of ion beam irradiation but results in a stable alignment layer so that the LCs retain their preferred alignment orientation.
The invention is a method for improving the anchoring of liquid crystals on a carbon alignment layer used in a liquid crystal display. The carbon alignment layer is a xe2x80x9cdiamond-likexe2x80x9d carbon (DLC) film which is formed either by a conventional process or by exposing a polymer film, such as polyimide, to Ar ions. The carbon alignment layer is exposed to hydrogen atoms which passivate the surface of the carbon layer to stabilize the anchoring of the subsequently deposited liquid crystals. The substrate on which the carbon layer is supported is located beneath a stretched tungsten filament, and the substrate and filament are located in a vacuum chamber containing hydrogen gas. The heating of the tungsten filament by an appropriate power source dissociates the hydrogen gas into hydrogen atoms and the hydrogen atoms contact the surface of the carbon layer. The process is applicable to stabilize carbon alignment layers that have been formed by directional deposition of carbon, as well as carbon alignment layers where the alignment is caused by a separate ion irradiation step after the carbon layer is formed. As a result of the hydrogen passivation, the liquid crystals subsequently deposited on the passivated carbon alignment layer retain their alignment substantially longer than without the passivation treatment.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.